AN0246 driving the analog inputs of a SAR AD converter

M AN246 Driving the Analog Inputs of a SAR A/D Converter Author: For the converter shown in Figure 1, the input signal could be ac, DC or both The operational amplifier is used for gain, impedance isolation and its drive capability A filter of some sort (passive or active) is needed to reduce noise and to prevent aliasing errors Bonnie C Baker Microchip Technology Inc INTRODUCTION Driving any A/D Converter (ADC) can be challenging if all issues and trade-offs are not well understood from the beginning With Successive Approximation Register (SAR) ADCs, the sampling speed and source impedance should be taken into consideration if the device is to be fully utilized In this application note we will delve into the issues surrounding the SAR Converter’s input and conversion nuances to insure that the converter is handled properly from the beginning of the design phase We will also review the specifications available in most A/D Converter data sheets and identify the important specifications for driving your SAR From this discussion, techniques will be explored which can be used to successfully drive the input of the SAR A/D Converter Since most SAR applications require an active driving device at the converter’s input, the final subject will be to explore the impact of an operational amplifier on the analog-to-digital conversion in terms of DC as well as ac responses A typical system block diagram of the SAR converter application is shown in Figure Some common SAR converter systems are Data Acquisition Systems, Transducers Sensing Circuits, Battery Monitoring applications and Data Logging In all of these systems, DC specifications are important Additionally, the required conversion rate is relatively fast (as compared to Delta-Sigma converters) and having a lower number of bits that are reliably converted is acceptable Amp Input Signal Source Filter Analog to Digital Converter Microcontroller Engine Filter Output DAC or PWM FIGURE 1: Block diagram of an application that has a SAR ADC in the signal path  2003 Microchip Technology Inc The ADC in Figure could be external or, in the case of a SAR converter, internal to the microcontroller The DAC / PWM block can be implemented internally or externally to the microcontroller as well This function is used to drive actuators, values, etc A filter following the DAC / PWM function is usually required to perform a smoothing function This filter would reduce glitch errors, quantization errors and provide drive or isolation to the actuator In this discussion we will focus on the input section to the A/D Converter BASIC OPERATION OF THE SAR ADC With the SAR ADC, the input signal should be considered in the DC as well as ac domain This is true even if you are only interested in a DC response DC Errors of the SAR ADC The offset and gain errors of an ADC can be easily calibrated out of the resulting data using the microcontroller at the output of the converter But the more difficult DC errors to calibrate out would be Integral Non-Linearity (INL) and Differential Non-Linearity (DNL) In most systems, these errors manifest themselves as incorrect conversions or noise Of the two specifications, INL is the “Holy Grail” of DC specifications because it describes the entire transfer function INL is a measure of how close to actual the transition points are to the ideal transfer function This is a difficult error to calibrate out with a microcontroller because every code needs to be evaluated for proper calibration and this error differs from device to device While noise is usually not a topic for DC accuracy, in this case it has merit It is important to realize that the SAR ADC operates in the frequency domain This is true even though you may think that you are measuring near DC signals If there is a noise source in the system, the “DC” conversion from sample-to-sample may not be the same This phenomena is reduced by using anti-aliasing filters When digitizing AC signals, other characteristics of the converter come into play These characteristics include distortion of the input signal and noise levels Anti-aliasing filters are also useful for these type of problems DS00246A-page AN246 Basic SAR ADC Operation Effects of Input Source Resistance At the input of a SAR ADC, the signal first sees a switch and a capacitive array, as shown in Figure The capacitors in this array are all connected to each other with the input signal node on one side and the noninverting input to a comparator on the other A detailed model of the internal input sampling mechanism of a SAR ADC is shown in Figure The critical values to pay attention to in this model are RS, C SAMPLE and RSWITCH C SAMPLE models the summation of the capacitive array shown in Figure Errors due to the pin capacitance and leakage are minimal The internal switch resistance combines with the external source resistance and sample capacitor to form a R/C pair This R/C pair requires approximately 9.5 time constants to fully change to 12-bits over temperature For the MCP3201 12-bit A/D Converter, 938 nsec are required to fully sample the input signal assuming R S ...AN246 Basic SAR ADC Operation Effects of Input Source Resistance At the input of a SAR ADC, the signal first sees a switch and a capacitive array, as shown in Figure The capacitors in this array... like the bandwidth of the operational amplifier and the harmonic distortion near the output rails are candidates for possible degradation of the signal as it travels through the data acquisition... starting to fall at 20 dB per decade In all cases, these areas are multiplied by the square root of the bandwidth of interest The different regions are added with a square root of the sum of the